Single cylinder internal combustion engines typically employ an intake valve and an exhaust valve for allowing fuel and air to enter the engine cylinder and allowing exhaust to exit the cylinder, respectively. These valves often are actuated by way of valve trains that impart linear movement to the valves in response to rotational movement of cams. In many such engines, the intake and exhaust valves are actuated in one direction (to close) by respective springs and actuated in the opposite direction (to open) by respective rocker arms. The rocker arms in turn are actuated by respective push rods that ride along respective cams that are supported by and rotate about a camshaft, which in turn is driven by a crankshaft of the engine. A fan also driven by the crankshaft blows air across the cylinder to cool the cylinder.
In such engines, it is important that oil or other lubrication be provided to at least the main bearings for the crankshaft and the camshaft, and that such oil be filtered. Consequently, most single cylinder engines also have carefully-designed lubrication systems to provide the necessary lubrication. The lubrication systems typically include an oil reservoir, a pump, and an oil circuit consisting of a series of passages by which oil is directed from the pump to the oil filter and to the components requiring lubrication. The oil passages are commonly manufactured by drilling or casting tubes into the crankcase and cover/oil pan of the engine.
Single cylinder engines of this design have several limitations. To begin with, the push rods that are positioned on such engines in between the camshaft and the rocker arms are positioned close together on a single side of the cylinder. Likewise, the pair of rocker arms at the cylinder head are positioned close together along a single side of the cylinder head, as are the pair of valves. Consequently, the valve bridge area of the cylinder head in between the valves, which is the hottest area of the cylinder head, is narrow and partially shielded from air being blown across the cylinder head by the fan. As a result, the valve bridge area may not be cooled as well as might be desirable, which can eventually cause weakening or breakage of the cylinder head, or to distortion/movement of the valve seats adjacent to this valve bridge area.
Additionally, the oil circuits in such single cylinder engines are often complicated in design and expensive to manufacture. In particular, the drilling or casting that is required in order to provide the required oil passages within the crankcase walls and cover/oil pan can be expensive and difficult to manufacture. The casting of tubular passages in particular is expensive insofar as it requires the use of cores or casting metal tubes within the engine material (e.g., aluminum). These drilling or casting (involving cores or metal tubes) procedures add to the complexity and costs of manufacturing the engine.
Further, given their complexity and large number of moving parts, the valve trains (including the camshaft and crankshaft) of such engines also can be difficult and costly to design and manufacture. For example, the two cams on a camshaft of such an engine typically must be oriented differently so that their respective main cam lobes are 100 or more degrees apart. Consequently, the manufacture of a camshaft with two such differently-oriented cams can be difficult and expensive, particularly when it is desired to integrally form the camshaft and cams as a single part. The costs of manufacturing of such valve train components can be further exacerbated if it is desired to manufacture such components from materials that are more durable or that provide quieter operation, since it is typically more difficult to mold or machine complex parts from such materials.
It would therefore be advantageous if a new single cylinder engine was designed that avoided or suffered less from the above problems. In particular, it would be advantageous if a single cylinder engine with robust, quietly-operating components could be designed that was more easily and cost-effectively manufactured than conventional engines, particularly in terms of the costs associated with the components of its valve train and lubrication system. Further, it would be advantageous if a single cylinder engine could be designed in which there was more effective cooling of the valve bridge area than in conventional engines.